The invention relates generally to the field of photovoltaic (PV) solar power systems, and more specifically to circuits and methods for protecting bypass diodes from overheating under partial shading conditions, and protecting human personnel from shock hazards.
FIG. 1 is a high level block diagram of a conventional solar power system 10 including a plurality of subunits 11 connected in series. Each subunit 11 includes: a first terminal 14 and a second terminal 15 for serial connection with other subunits 11; a bypass diode 13; and a PV segment 12 comprising a plurality of PV cells wired in series. The subunits feed power to an inverter 16 which produces an ac voltage output 17 that is typically tied to the ac electrical power grid.
The PV segments 12 in a solar power system, such as 10, are typically well matched in their characteristics, so when all the segments are equally illuminated by sunlight, they produce nearly equal amounts of energy. However, occasionally one PV segment will be partially shaded by some obstruction—such as a tree branch or chimney—and consequently, the energy produced by the shaded segment is reduced. In this situation, the unshaded segments can force the shaded segment into reverse breakdown, wherein the PV cells generate excessive heat, and may be damaged. The bypass diode 13 protects the shaded PV segment 12 from reverse breakdown by allowing the excess current to flow around the PV segment, rather than through it.
A potential problem in PV systems, such as 10, is overheating in one or more of the bypass diodes 13. For example, assume the string current (ISTRING) is 11 Amps, but the short-circuit output current of one of the PV segments 12 is only 1 Amp, due to shading. This means the current in the associated bypass diode 13 is 10 Amps. If the forward voltage drop of the bypass diode 13 is 0.5V, then the heat dissipation in the bypass diode is 5 W. A typical solar junction box affixed to the back side of a solar power module contains three bypass diodes, so in this example the total power dissipation in such a junction box could be as high as 15 W if all three PV segments are shaded. Furthermore, ambient temperatures around a solar junction box can easily exceed 70 C. Give all these factors, the junction temperature of a bypass diode 13 can exceed 200 C. When the shade is removed, the short-circuit output current of the PV segment 12 increases, and the associated bypass diode 13 becomes reverse biased. However, because the diode junction is still very hot, the reverse leakage current may be excessive, particularly if the diode is a Schottky. For example, if the reverse leakage is 200 mA, and the output voltage of the PV segment 12 is 12V, then the diode 13 dissipates 2.4 W. But leakage current approximately doubles for every 10 C. rise in junction temperature, so as the diode 13 becomes hotter, the leakage increases, which heats the diode even more. This positive feedback mechanism—known as thermal runaway—can easily destroy the diode 13, typically making it a short circuit.
One solution well known to those of ordinary skill in the art, is to use an active bypass circuit. This approach is becoming increasingly popular, with several products already on the market at the time of this writing. Examples of such are the LX2400 from Microsemi Corp. of Garden Grove, Calif. (US), and the SPV1001T40 from STMicroelectronics of Agrate Brianza (MI) (IT).
FIG. 2 shows a high level block diagram of a subunit 20 including a typical active bypass circuit comprising a bypass switch 21 and a local controller circuit 22. The local controller 22 includes positive 23 and negative 24 inputs for sensing the polarity of the voltage across the bypass diode 13. When the PV segment 12 is partially shaded, the bypass 13 diode becomes forward biased. The local controller 22 sees a positive voltage across its differential inputs 23 and 24, and responds by closing the bypass switch 21. The voltage drop across the closed bypass switch 21 is much lower than the voltage drop across the diode 13 before the switch was closed. Accordingly, the heat dissipation is greatly reduced. When full sunlight is restored to the PV segment 12, current in the bypass switch 21 reverses. The local controller 22 sees a negative voltage between 23 and 24, and responds by opening the bypass switch 21.
One problem with the prior art circuit 20 is that the local controller 22 needs a voltage supply to operate. The bypass switch 21 is typically a power MOSFET that requires at least 5V applied between its gate and source terminals to fully turn on. As such, the local controller 22 needs a supply voltage of at least 5V. The PV segment 12 typically provides 12V in full sunlight, but when the bypass switch 21 is closed, the outputs of the PV segment 14 and 15, are essentially shorted together so the PV segment 12 cannot provide the voltage needed to power the local control circuit 22.
The bypass switch 21 does not have zero resistance, so when current flows through the switch 21, a small voltage—typically 50 mV—develops between the terminals 14 and 15. Various examples of prior art have sought to utilize this small voltage to create a supply voltage of 5V or more. For example, U.S. Patent Application Pub. No. 2011/0006232 A1 discloses a self-powered active bypass circuit wherein a system of charge pumps and oscillators amplifies the voltage, and U.S. Patent Application Pub. No. 2011/0242865 A1 (the '865 application) discloses a self-powered active bypass circuit utilizing resonance for amplification. But each of these prior art examples controls only a single bypass switch, while a typical PV solar power module requires three or more such bypass switches. Furthermore, these examples of prior art are relatively expensive; for example, the '865 application requires a relatively expensive transformer for each bypass circuit.
Another problem with the conventional system 10 is safety for installer personnel and firefighters. An interrupter switch 18 is typically used to shut down the system 10, but such a switch 18 merely shuts off the flow of current (ISTRING) into the inverter 16. The problem is that the array continues to produce a high voltage (VSTRING) that can be several hundred volts, posing a shock hazard to anyone who connects or disconnects the cables.
Therefore, there is a need in the solar power industry for a solar power module that protects the bypass diodes from overheating at low cost, and reduces the risk of shock for installers and firefighters.